A voltage regulator having an overcurrent protection circuit limits an output current to prevent the voltage regulator from being burned when the output current exceeds an overcurrent value. In addition to the function of limiting the output current to be below the overcurrent value, the voltage regulator with a fold-back overcurrent protection circuit reduces the output current when an output voltage is lowered. Therefore, it prevents the voltage regulator from being burned and reduces power consumption.
Please refer to FIG. 1A and FIG. 1B. FIG. 1A is a circuit diagram of a voltage regulator with a drooping type overcurrent protection circuit. FIG. 1B is a graphical representation explaining the output voltage and the output current of the voltage regulator with the drooping type overcurrent protection circuit. As shown in FIGS. 1A and 1B, a constant voltage power circuit comprises a reference voltage 10, an output transistor 12, an error amplifier 11, a resistor 13, and a resistor 14.
The resistor 13 and the resistor 14 are connected in series between an output voltage (Vout) and a ground voltage to form a voltage divider circuit. Therefore, a voltage Va at a node A is proportional to the output voltage (Vout). The voltage Va at the node A is a feedback signal to be inputted into the error amplifier 11. The error amplifier 11 amplifies the difference voltage between the reference voltage 10 and the voltage Va. An output terminal of the error amplifier 11 generates an amplified difference voltage and controls the output transistor 12. Therefore, the output voltage (Vout) has a constant value. The detailed operation method will be illustrated as follows.
In normal operation, the voltage Va at the node A is close to the reference voltage 10. When the voltage Va at the node A is lower than the reference voltage 10, the amplified difference voltage decreases. Consequently, a gate-source voltage of the output transistor 12 increases and the on resistance decreases. Therefore, the output voltage (Vout) increases. Conversely, when the voltage Va at the node A is higher than the reference voltage 10, the amplified difference voltage increases. Consequently, the on resistance of the output transistor 12 increases and the output voltage (Vout) decreases. Therefore, in a case that the voltage Va is used as the feedback signal and inputted into the error amplifier 11, the constant voltage power circuit may be controlled to generate the constant output voltage (Vout).
In order to prevent the output current through the output transistor 12 from being too high, the voltage regulator usually includes the overcurrent protection circuit. As shown in FIG. 1A, the overcurrent protection circuit comprises a transistor 15, a resistor 16, a transistor 17, a resistor 18 and a transistor 19.
As shown in FIG. 1A, a gate of the output transistor 12 and a gate of the transistor 15 are connected with each other, so that there is a fixed ratio between a current flowing through the transistor 15 and the output current. The fixed ratio is based on the size of the output transistor 12 and the transistor 15. Obviously, when a load resistor 20 decreases, the output current increases. When the higher the output current, a current through the resistor 16 is higher and a gate voltage of the transistor 17 is higher.
When the output current reaches the overcurrent value, the gate voltage of the transistor 17 is higher than the threshold voltage. The transistor 17 is turned on and a current flows through the resistor 18. Therefore, the transistor 19 is turned on, the gate voltage of the output transistor 12 increases, and then the output transistor 12 is turned off. Since the output voltage (Vout) of the constant voltage power circuit decreases, the overcurrent protection mechanism is formed.
As shown in FIG. 1B, when the output current reaches the overcurrent value, the overcurrent protection circuit is enabled and the output voltage (Vout) decreases quickly. As shown in FIG. 1B, when the output voltage (Vout) decreases, the output current is still very high. Such overcurrent protection circuit is called a drooping type overcurrent protection circuit.
U.S. Pat. No. 7,233,462 discloses a voltage regulator with a fold-back overcurrent protection circuit. Please refer to FIG. 2A and FIG. 2B. FIG. 2A is the voltage regulator with a fold-back overcurrent protection circuit in the related art. FIG. 2B is a graphical representation explaining the output voltage and the output current of the voltage regulator with the fold-back overcurrent protection circuit.
In comparison with the overcurrent protection circuit in FIG. 1A, the overcurrent protection circuit in FIG. 2A further includes a transistor 2, a transistor 3 and a transistor 1. A gate of the transistor 2 is connected to a drain the transistor 2, and the gate of the transistor 2 is connected to a drain of the transistor 1. A gate of the transistor 1 is connected to the node A. A source of the transistor 1 is connected to the ground terminal. A gate of the transistor 3 is connected to the gate of the transistor 2. A drain of the transistor 3 is connected to a drain of the transistor 17 and a gate of the transistor 19. The transistor 2 and the transistor 3 are collectively defined as a current mirror circuit.
When the output voltage Vout of the constant voltage power circuit is normal, the voltage Va at the node A is higher than the threshold voltage of the transistor 1. Consequently, the transistor 1 is turned on, and a current will flow through the transistor 2. At the same time, the transistor 3 will generate the same current value.
When a short circuit occurs at the output terminal, the output voltage Vout decreases and the output current increases. Thus, a current flowing through the transistor 15 increases and the voltage Va decreases. A current flowing through the resistor 16 increases and the gate voltage of the transistor 17 increases. Obviously, if the gate voltage of the transistor 17 exceeds the threshold voltage, the transistor 17 is turned on. When a generated current upon the transistor 17 starting up exceeds a current flowing into the transistor 3, a gate voltage of the transistor 19 decreases and a gate voltage of the transistor 12 increases. Therefore, the overcurrent protection circuit limits the output current.
In other words, when the overcurrent protection circuit operates, the decreased output voltage causes the gate voltage (Va) of the transistor 1 to decrease, so that a current flowing into the transistor 2 is also suppressed. Because the transistor 3 and the transistor 2 form the current mirror circuit, the current flowing through the transistor 3 is also suppressed.
Please refer to a curve I in FIG. 2B, when the short circuit occurs and the output current exceeds an overcurrent value, the output current decreases while the output voltage decreases. Thus a fold-back overcurrent protection circuit is generated.
However, due to a semiconductor process deviation, the actual resistance values of the resistor 13, the resistor 14 and the resistor 16 are very different from the designed resistance values. Since the resistor 13 and the resistor 14 are collectively defined as a voltage divider circuit, even if they can not obtain accurate resistance values, the ratio of the resistor 13 and the resistor 14 is not changed by the semiconductor process deviation. Thus, the semiconductor process deviation does not make a great impact on the voltage divider circuit.
The resistance value of the resistor 16 makes a great impact on the entire overcurrent protection circuit. Since a voltage drop of the resistor 16 is used to control the protection circuit startup or not, when the actual resistance value of the resistor 16 is higher than the designed resistance value, the curve of the output voltage and the output current of the regular becomes a curve II. On the contrary, when the actual resistance value of the resistor 16 is lower than the designed resistance value, the curve of output voltage and the output current of the regular becomes a curve III.
In other words, the change of the resistance of the resistor 16 makes the startup time of the overcurrent protection circuit different. Thus, the curve of the output voltage and the output current of the regular may be varied between curve II and curve III. It causes the application of the voltage regulator problem.
U.S. Pat. No. 7,183,755 also revealed another type of voltage regulator with a fold-back overcurrent protection circuit. Please refer to FIG. 3, the voltage regulator with the fold-back overcurrent protection circuit 100 includes a constant voltage power circuit 101 and an overcurrent protection circuit 102.
The constant voltage power circuit 101 includes: a reference voltage 111, an output transistor M101, an error amplifier AMP, a resistor R101 and a resistor R102. The resistor R101 and the resistor R102 are connected in series between an output voltage terminal (OUT) and a ground voltage to form a voltage divider circuit. Therefore, a divided voltage VFB is proportional to an output voltage Vo. The divided voltage VFB is a feedback signal to be inputted into the error amplifier AMP. The error amplifier AMP amplifies a difference voltage between the reference voltage 111 and the divided voltage VFB. An output terminal of the error amplifier AMP generates an amplified difference voltage and controls the output transistor M101. Therefore, the output voltage Vo is a constant value.
The overcurrent protection circuit 102 includes PMOS FET (field-effect transistor) devices M102, M103, M106, M107, depletion-type NMOS FET devices M104, M105, a resistor R103, a bias current source 112 and an offset voltage Vof. If an output current (io) is less than an overcurrent value of the overcurrent protection circuit 102, a drain current of transistor M102 is relatively small and flows through the resistor R103. Therefore, a drain voltage of the transistor M105 can not turn on the transistor M105. Meanwhile, the drain voltage of the transistor M105 is almost equal to an input voltage (Vin). Consequently, the transistor M103 fails to be turned on, and the overcurrent protection circuit 102 does not start up.
If the output current (io) is higher than the overcurrent value of the overcurrent protection circuit 102, the overcurrent protection circuit 102 starts up. Meanwhile, a voltage drop of the resistor R103 is higher than the threshold voltage of the transistor M105 to turn on the transistor M105. The decreasing drain voltage of the transistor M105 causes the transistor M105 to be turned on, and then a gate voltage of the transistor M101 increases. An output current from transistor M101 decreases and the output voltage decreases. Therefore, the overcurrent protection mechanism is formed.
Similarly, the voltage drop of the resistor R103 controls the overcurrent protection circuit 102 to start up or not. However, the process deviation causes an error of the resistor R103, and thus the overcurrent value fails to be confirmed. Therefore, it makes the startup time of the overcurrent protection circuit different. It causes problems in the applications of the voltage regulator.
Please refer to FIG. 4, it revealed another type of voltage regulator with a fold-back overcurrent protection circuit. Wherein, the voltage regulator with the fold-back overcurrent protection circuit 200 includes a constant voltage power circuit 220 and an overcurrent protection circuit 230.
The constant voltage power circuit 220 includes: a reference voltage 11, an output transistor M1, an error amplifier A1, a resistor R1 and a resistor R2. The resistor R1 and the resistor R2 are connected in series between an output terminal (OUT) and a ground voltage to form a voltage divider circuit. Therefore, a divided voltage VFB is proportional to an output voltage Vo. The divided voltage VFB is a feedback signal to be inputted into an error amplifier A1. The error amplifier A1 amplifies a difference voltage between the reference voltage 211 and the divided voltage VFB. An output terminal of the error amplifier A1 generates an amplified difference voltage and controls the output transistor M1. Therefore, the output voltage Vo is a constant value.
The overcurrent protection circuit 230 includes PMOS FET (field-effect transistor) devices M2, M3, M6, NMOS FET devices M4, M7, M8, resistors R3, R4, bias current source 212 and an offset voltage Vof. Wherein, M5, M6, M7, M8, bias current source 212 and the offset voltage Vof forms a differential amplifier A2. If an output current (io) is less than an overcurrent value of the overcurrent protection circuit 230, a drain current of transistor M2 is relatively small and flows through resistor R3. Therefore, a drain voltage of the transistor M6 can turn on the transistor M6, and the transistor M5 is turned off. Meanwhile, the drain voltage of the transistor M5 is close to the ground voltage, so that the transistor M4 fails to be turned on and the drain voltage of the transistor M3 is equal to a input voltage (Vin). Meanwhile, the overcurrent protection circuit 230 does not start up.
If the output current (io) is higher than the overcurrent value of the overcurrent protection circuit 230, the overcurrent protection circuit 230 starts up. Meanwhile, a voltage drop of the resistor R3 causes the transistor M6 to be turned off and the transistor M105 to be turned on. Since the transistor M7 and the transistor M4 are collectively defined as a current mirror circuit and turned on at the same time, the transistor M3 is turned on and a gate voltage of the transistor M1 is close to a input voltage (Vin). Meanwhile, an output current from the transistor M1 decreases, and the output voltage decreases. Therefore, the overcurrent protection mechanism is formed.
Similarly, the voltage drop of the resistor R3 controls the overcurrent protection circuit 230 to start up or not. However, the process deviation causes an error of the resistor R3, so that the overcurrent value fails to be confirmed. Therefore, it makes the startup time of the overcurrent protection circuit 230 different. It causes problems in the applications of the voltage regulator.
Accordingly, it is an object of the invention to provide a regulator with a fold-back overcurrent protection circuit for improving the related issue about semiconductor process deviation which causes the overcurrent value uncertainty, and at the same time setting a minimum output current of the voltage regulator when a short circuit occurs.